1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for coordinating dimmer compatibility functions.
2. Description of the Related Art
Electronic systems utilize dimmers to modify output power delivered to a load. For example, in a lighting system, dimmers provide an input signal to a lighting system, and the load includes one or more light sources such as one or more light emitting diodes (LEDs) or one or more fluorescent light sources. Dimmers can also be used to modify power delivered to other types of loads, such as one or more motors or one or more portable power sources. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level. For example, some analog based dimmers utilize a triode for alternating current (“triac”) device to modulate a phase angle of each cycle of an alternating current (“AC”) supply voltage. “Modulating the phase angle” of the supply voltage is also commonly referred to as “chopping” or “phase cutting” the supply voltage. Phase cutting the supply voltage causes the voltage supplied to a lighting system to rapidly turn “ON” and “OFF” thereby controlling the average power delivered to the lighting system.
FIG. 1 depicts a lighting system 100 that includes a leading edge dimmer 102. FIG. 2 depicts exemplary voltage graphs 200 associated with the lighting system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VSUPPLY from voltage supply 104. The supply voltage VSUPPLY, indicated by voltage waveform 202, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. A leading edge dimmer phase cuts leading edges, such as leading edges 204 and 206, of each half cycle of supply voltage VSUPPLY. Since each half cycle of supply voltage VSUPPLY is 180 degrees of the supply voltage VSUPPLY, a leading edge dimmer phase cuts the supply voltage VSUPPLY at an angle greater than 0 degrees and less than 180 degrees. Generally, the voltage phase cutting range of a leading edge dimmer 102 is 10 degrees to 170 degrees. The leading edge dimmer 102 can be any type of leading edge dimmer such as a triac-based leading edge dimmer available from Lutron Electronics, Inc. of Coopersberg, Pa. (“Lutron”). A triac-based leading edge dimmer is described in the Background section of U.S. patent application Ser. No. 12/858,164, entitled Dimmer Output Emulation, filed on Aug. 17, 2010, and inventor John L. Melanson.
Ideally, by modulating the phase angle of the dimmer output voltage Vφ—DIM, the leading edge dimmer 102 effectively turns the constant current lamp 122 OFF during time period TOFF and ON during time period TON for each half cycle of the supply voltage VSUPPLY. Thus, ideally, the dimmer 102 effectively controls the average power supplied to the constant current lamp 122 in accordance with the dimmer output voltage Vφ—DIM. However, in many circumstances, the leading edge dimmer 102 does not operate ideally. For example, when the constant current lamp 122 draws a small amount of current iDIM, the current iDIM can prematurely drop below a holding current value HC before the supply voltage VSUPPLY reaches approximately zero volts. When the current iDIM prematurely drops below the holding current value HC, a triac-based leading edge dimmer 102 prematurely resets, i.e. prematurely disengages (i.e. turns OFF and stops conducting), and the dimmer voltage Vφ—DIM will prematurely drop to zero. An exemplary premature reset would occur if the dimmer 102 reset at time t3 and the dimmer voltage Vφ—DIM dropped to 0V at time t3. When the dimmer voltage Vφ—DIM prematurely drops to zero, the dimmer voltage Vφ—DIM does not reflect the intended dimming value as set by the resistance value of variable resistor 114. The diode for alternating current (“diac”) 119, capacitor 118, resistor 116, and variable resistor 114 form a timing circuit 116 that resets triac 106. Additionally, the triac 106 of leading edge dimmer 102 can reset and then conduct repeatedly, i.e. disengage (non-conductive), reengage (conductive), disengage (non-conductive), and so on repeatedly during a half-cycle of supply voltage VSUPPLY when the current iDIM is below or near the holding current value HC. A “reset-conduct” sequence occurs when the dimmer 102 resets and then conducts the supply voltage VSUPPLY one or more times during a single half-cycle of the supply voltage VSUPPLY.
The lighting system 100 includes a resistor, inductor, capacitor (RLC) network 124 to convert the dimmer voltage Vφ—DIM to an approximately constant voltage and, thus, provide an approximately constant current iOUT to the constant current lamp 122 for a given dimmer phase angle. Although relatively simply to implement, the RLC network 124 is inefficient because of, for example, resistor-based power losses. Additionally, reactive load presented by the RLC network 124 to the dimmer 102 can cause the triac to malfunction.